Electroperforation of sheet material



Oct. 30, 1945. L W MEAKER ET AL 2,388,069

ELECTRO-PERFORATION OF SHEET MATERIAL Filed July 24, 1942 2 sheets-sheet1 Oct. 30, 1945v J. w. MEAKER ET Al..

ELECTRO-PERFORATION OF SHEET MATERIAL Filed July 24, 1942 2 Sheets-Sheet2 QXS Patented Oct. 30, 1945 ELECTROPERFORATION F SHEET MATE RIAL

John W. Meaker, Evanston, and Edward H. Yonkers, Jr., Glencoe, Ill.;said Yonkers, Jr., assigner to said Meaker Application July 24, 1942,Serial No. 452,142

(Cl. P25- 265) Claims.

This invention relates to the perforation of sheet material, havingdielectric properties, by means of electric arcs. The principles of theinventlon may be utilized in connection with the electro-perforation ofpaper, leather, leatherette, patent leather, rubberized fabrics, puregum rubber sheeting, or of any other dielectric sheet material. Thematerial may be treated, in accordance with the present invention, asseparate sheets or as a continuous web or strip. The material may be inthe form of single sheets, strips or Webs or in the form of amultiplicity of superposed sheets, strips, webs or plies, to be piercedsimultaneously. l

Any suitable apparatus may be used, in carrying out the presentinvention, for producing the electro-perforation of the sheet material.A particularly suitable apparatus for the purpose is disclosed in UnitedStates Patent of John W. Meaker, No. 2,372,508, issued March 27, 1945;by means of which apparatus a large number of closely spacedperforations may be made through the sheet material per unit areathereof (provided the latter is not too highly resistant to thepotential arcs) with a considerable degree of uniformity ofdistribution, and with the sheet material moving at relatively highspeed; high speed through the perforating machine being in some casesimportant on the grounds of economical operation, or, specifically tosynchronize the periorating operation with the movement of the sheetmaterial through other apparatus, such, for example, as bag makingapparatus, in the production of perforated paper bags.

Under these, and in fact even under less exisent conditions, it has beenfound dicult to perorate certain types of sheet material havinghighdielectric'strength, such as leatherette, rubberized fabrics, or evenpaper, especially when treated in multiplel plies. At least it has beenfound difficult in the case of some materials to form at high speed arelatively large number of perforations per unit area evenly distributedover the given area.. En the case of some materials the dielectricstrength is such as to prevent the discharges from taking place unlessexcessively high potentials are used; or if any perrforations are made,there is a tendency, as the sheet is moved through the spari; gaps, forthe discharges to pass through perforations already made instead ofmaking new perforations. This may happen several times at any spark gap,depending upon the frequency of the discharges, the voltage of thecurrent used, the dielectric strength of the sheet material, and thespeed of movement of the material through the spark gaps. M a result,the perforations for a given area may be fewer in number, less uniformin size, and less uniformly spaced than is desired.

The present invention overcomes all of these diiliculties by subjectingthe sheet material to a treatment, while or before being subjected tothe electro -perforating operation, whereby the resistance to theelectric discharges is reduced at certain denite points on the sheetmaterial so that electro-perforation will take place at these points.Preferably this is accomplished by forming the material with pin pricks,surface breaks or ruptures (which will be referred to as pointincisions" and which may or may not be complete perforations), atdefinite places at which it is intended to have the electric arcs passthrough and pierce the material. This procedure has several advantages:first, it reduces the dielectric strength of the material treated at theintended points of perforation, making it possible to satisfactorilyelectro-perforate material which has previously been found impossible,impractical or at least dilicult to perforate electrically; and, second,it tends, in all materials, to prevent re-arcing through perforationsalready made and thereby makes it possible: (l) to increase the numberof periorations made per unit area; (2) to give the perforations uniformspacing or approximately any desired arrangement or pattern; (3) toincrease the speed at which the material can be put through theperforating machine to eect the desired close spacing; and (d) to givethe perforations more nearly uniform sise. By a suitable arrangement ofthe point incision devices and a corresponding arrangement of the spari:gaps, together with proper control of the timing of the electricalimpulses impressed against said gaps, various distribution patterns ofperforations may ce produced. Particularly it is possible to bring aboutclose spacing of perforaticns over a large area of the material in ashort period of time.

The phrase point incisions is used to include holes, pin pricks, orother surface breaks or ruptures Whether extending completely throughthe material cr nut. The incisions may, in case of some materials, bemerely momentaryfclosing or partially closing as soon as made. Theoperation may involve, as in pin pricking in the case of some fabrics,only the pushing apart of the materials at given points. But, anyphysical disturbance of or change in the character of the material atdeflhite points, such, for example by ihclentatlons, without actualrupture, of such character that the dielectric strength of the materialis reduced at such points, is within principle of operation of thepresent invention.

The operation may take place either before electro-perforation orsimultaneously therewith.

The former method is specifically described herein and is preferred.

Preferably the reduction of dielectric strength at definite points isaccomplished by mechanical means, as for example, by pins or otherpuncturing devices, which for convenience may be fixed to a rotatingdrum or to rotating discs, and with ".which the sheet material comesinto contact before it passes intov the perforating machine. However, itmight be effected otherwise, for example by spotting the material with asolvent or other chemical agent. The preferred -mechanical operation maybe described as a pin-pricklng operation since the point incisions areintended to be, ordinarily, quite minute. Theoretically, and for thebest results, the arrangement of the spark gaps of theelectro-perforating machine and the frequencies of the electricalimpulses produced at said gaps, relative to the speed of movement ofthe. sheet material through the apparatus, should be in conformity withthe distribution of point incisions made in the sheet material, so

that the arcing will take place at, or approximately at, the points ofsuch incisions. In this way a definite or approximately definite patternor distribution of perforations may be produced. -For example, theperforations may be distributed over the entire area of the sheet atsubstantially equal distances from each other, both in the direction ofmovement of the sheet and transversely thereof. The point incisions notonly reduce the dielectric strength which the potential has to overcome,making possible the electro-perforation of fabrics which have been founddimcult. if not impossible, to perforate electrically, but alsodetermine, in the case of all fabrics, at least approximately, thepattern distribution of the perforations when formed.4

'I'he pin-pricking, or equivalent operation, may completely perforatethe material even when a single sheet of material is treated. However,this will ordinarily not be necessary nor desirable. In the treatment ofa plurality of superposed plies of sheet material, paper for example,the pin pricking operation may be performed so as to perforate one ormore of the' plies; and this may be necessaryk or desirable when, in themanufacture, for example, of heavy duty multi-wall paper bags, the pliesare so numerous or the paper so heavy as to make their perforation byelectrical discharges impossible or dimcult except with excessive, per1haps. dangerously high, voltages. the pin pricks extend completelythrough the sheet material, the mechanically made perforations thusproduced will be changed to electric arc made perforations, by action ofthe electric arc, and their chara'cter radically changed.

Punched, r pin pricked perforations, particularly if minute, involve,necessarily, burrs or frayed :edges around the openings. When the sheetmaterial, the paper sheet of a bag for example, is subjected to pressureor other manipulation, the mechanically made perforations therein tendto close, wholly or partially, so that the ventilating function of suchperforations is impaired or made uncertain. Moisture may also, dependingupon the characterof the sheet material, tend to-close or partiallyclose mechanically made perforations.

Electric arc formed perforationsare burrless. The holes are made by theburning away of minute portions of the material. The edges of thematerial around 4the holes are, in some cases, seared, fused orplasticized, and in all ca'ses are more or less clearly dei'lned indistinction to the frayed edges or burrs characteristic of mechanicallymade perforations. Therefore, electric arc made perforations arepermanent under all conditions and their Ventilating-.functionunimpaired by pressure, moisture or other conditions.

In any case, if i When the pin pricking operation perfor-stes the sheetmaterial, giving burred edges. thev burrs will be burned away and theedges possibly fused (depending on the character of the material) by theelectric arcs. Y

The electric arc formed perforations may be very minute, microscopic oreven sub-microscopic or they may be large enough to be visible to thenaked eye. 'I'his will depend upon the intensity and duration of thecurrent flow through the perforation after the high tension dischargetakes place. Also with high frequency impulses the perforations may beincreased in diameter by repeated discharges through the sameperforations'. In the case of material consisting of a more or lessloosely woven textile fabric covered with a dense enamel coating of highdielectric strength the perforations' may in some cases extend onlythrough the coating,l and not through, or entirely through, the fabricbacking, at least as clearly defined perforations; and this. in factwill be all that is necessary for Ventilating purposes, provided thebacking has a sufciently loose weave. 'Ihe tendency however is alwaysfor the electric dscharge to burn more or less clearly defined holesthrough the entire thickness of the fabric The invention is illustratedin a preferred embodiment in the accompanying drawings wherein:

Fig. 1 is a somewhat diagrammatic plan view of an apparatus suitablefor` carrying out the present invention.

Fig.2 is a cross sectional view in a plane passing through the set oftransversely-arranged spark gaps adjacent to the pin pricking apparatus.

Fig. 3 is a longitudinal sectional view which may be considered asdefined by a line on Fig. 1 passing through the left-hand pin prickingdisc and left-hand spark gap of the rst group of 'spark gaps, and thenjogged twice so as to pass.

and third 'sets of spark gaps successively.

Fig. 4 is a section, of very much exaggerated thickness, through amaterial diilicult to perforate by ordinary methods but readilyperforated in accordance with the present invention--a material such,for example, a-s leatherette, patent leather or rubberized fabric.

Fig. 5 is a similar View of the material after being initially pinpricked; and

Fig. 6 is a similar view of the material after electrical perforation.

Referring to the drawings, the sheet of material treated is indicated byletter A. It may consist of a fabric backing lb and a dense, highlydielectric, surfacing compound il (Figs. 4, 5 and 6). Leatherette is afabric of this sort; and the density and composition of the coating orsurfacing material makes such fabric very diilicult to perforateelectrically by methods heretofore known. Other fabrics diiicult toperforate by the electric `arc method because of their high dielectricpear to close up as soon as the pins are with drawn. Nevertheless, thedielectric strength has been reduced by the pin pricking operation so asto provide paths of reduced resistance for the electric arcs.

Referring again to the drawings, B designates,-

generally, the pin Drinking device, and C the elec- 5 tro-perforatingmechanism. The' sheet of material A may be moved, continuously, andguided through the pin pricking apparatus B, and then through the sparkgaps of the electro-perforating machine by feed rollers I2, I2 and guiderollers I3, I3. As shown, the pin pricking apparatus B consists of asupporting roller I4 which may, if desired, be spring mounted, and of aplurality of discs I5, |51, |52, |53, |54, |55, |56, I5", I5a on shaftI6, which discs are provided, in circumferential arrangement, withpointed pins I1. The discs may be either xed to or rotatable on shaft Bwhich latter may, if the discs are fixed thereto, be revolved either bydriving means or by contact with the moving sheet material. The pins, inthe arrangement shown, do not perforate the sheet of material, althoughthey might do so, but merely make surface incisions, breaks orindentations therein which are indicated at a' Figs. 1, 2, 3 and 5. Theshaft may be mounted so as to be vertically adjustable for fabrics ofdifferent thicknesses.

'Ihe electro-perforating apparatus C comprises, as shown in thedrawings, three sets of electrodes, each set consisting of three pairsof opposed electrodes in series with each other and with the secondarycoils of three transformers, respectively. The number of electrodes ineach set may, of course, be increased or decreased; and also the numberof sets of electrodes. The pairs of electrodes of one set are preferablystaggered with respect to the pairs of electrodes of the other sets. Thepairs of electrodes are approximately in line with the pin prickingdiscs |5|58.

Referring particularly to Figs. l, 2 and 3, the pairs of electrodes ofthe first set of electrodes are designated I8, I8', I9, I9' and 20, 20.They are shown as supported by bars of insulating material 2|, 2| andare connected by wires 22, in series with each other and with thesecondary coil 23 of a transformer, designated D, as a whole, theprimary coil of which 24 is connected to the supply leads 25.

In the arrangement shown, the electrodes I8, I3 are aligned with thedisc I5; the electrodes I9, I9' with the disc |53; and the electrodes20, 20' With the disc |56. In the neXt set of e'lectrodes, designated 26as a whole, the electrodes 21, 28 and 29, and, of course, thecomplementary electrodes beneath the material treated, are in line withthe discs |51, |54 and |57, respectively. These electrodes are in serieswith each other and with the secondary coil 23 of the transformer E. Theelectrodes 30, 3| and 32, and their complementary electrodes, formingthe third set of electrodes 33, are in line with the discs |52, |55 and|58, respectively, and are in series with each other and with thesecondary coil 23 of the transformer F. Transformers E and F areconnected to the supply leads 25 so that the transformers D, E and F arein parallel.

By this arrangement, and by suitable control of the timing of theelectric discharges relative to the speed of movement of the sheet ofmaterial treated, the point incisions or pin pricks a made by the discs|5-|58 are brought into or near the gaps between the electrodes in suchtime relation with the electrical impulses in the electrode circuitsthat the discharges arc through the material at the pin pricks a so thatsaid pin pricks are, by action of the impulses, formed into electric arcmade perforations b. The reason for having a plurality of sets ofstaggered spark gaps, instead of a single setcf spark gaps arrangedclose together, is that the latter arrangement involves the danger ofthe current jumping from one electrode to an adjacent electrode on thesame level instead of across the intended spark gaps and through thesheet material moving through said gaps. There are also mechanicaldifficulties in arranging the pairs of electrodes too close together.

The Iperforating apparatus may employ direct currents, interrupted atthe proper intervals by suitable make and break devices. Or theapparatus may be supplied from an alternating current source, such as agenerator, controlled so that discharges will take place between opposedelectrodes while the point incisions are between or near to saidelectrodes. Obviously, the frequencies in the case of alternatingcurrents may be higher than required by the point incision pattern. Thatis, several alternations of current may take place through the sameperforation while the sheet of material is in motion. In some cases thecurrent may be interrupted by the continued movement of the sheet andconsequently several alternations of potential may take place withoutany discharges through the material. In such cases discharges may occurat points of low dielectric strength other than at the incision points.This may be regarded as accidental.

For mechanical reasons it is desirable to employ frequenciessufficiently high so that synchronism with the point incision pattern isnot essential.

While the invention has been described in a preferred embodiment, itwill be understood that it is not to be considered as so limited. Theintention is to cover all equivalents and also all modifications Withinthe scope of the hereto appended claims.

While the apparatus, as shown and described, is designed so as toprovide a distribution of perforations over the entire area of the sheetof material treated, with the `perforations substantially equi-distantfrom each other and close together, and it is one of the objects of theinvention to make practical such distribution of perforations throughmaterial diicult to perforate electrically in such manner, the inventionis not limited thereto. Obviously, by proper design and arrangement ofthe pin pricking devices and of the perforating electrodes, variouspatterns of perforations may be produced. For the purpose of simplifiedshowing, only three sets of electrodes are illustrated. The number maybe increased to ve or six or more, while maintaining the staggeredrelation between all sets, with the result that the perforations will becloser together, transversely of the sheet, than is indicated in Fig. 1.

In case frequencies are employed higher than that represented by thetime required to move the sheet the distance between pin pricks l whichis desirable for the reasons above stated), and in case the dielectricstrength of the material, as a Whole or in places, be such as to permitperforation at points other than at the pin pricked points, theperforation pattern indicated in Fig. l might, as stated, be modied byadditional perforations irregularly distributed with reference to theintended pattern. In most instances this would be a matter of noimportance and might be an advantage because resulting in the productionof a larger number of perforations per unit area than would otherwise bethe case. The expedient of pin pricking the material at denite pointswould insure, in any event, a desired minimum number of perforations perunit area. In many cases this is the most important desideratum. If

each unit area be assured of being perforated with a minimum number ofperforations, a few more perforations would ordinarily involve nodisadvantage. In other words, a definite pattern may be produced ifdesired, or a minimum of perforations perunit area may be assured withthe possibility of additional, irregularly distributed perforations, inaccordance with the character of the material treated and the manner inwhich the apparatus is designed and controlled. In any case, the arcs, rsome of them, are directed through the material treated at definitepoints at which the dielectric strength of the material has been reducedby the pin pricking operation. This mode of operation makes possible,first, the electro-perforation of materials, such as leatherette forexample (or other difilcultly perforable materials mentioned above), thedielectric strength of which is such as to make electro-perforation, bymethods heretofore used, impossible or impractical, at least when alarge number of perforations closely spaced together is the requirement;second, it makes possible the certainty that, for any type of fabric, aminimum number of perforations will be made per unit area; and third, itmakes possible a distribution of the perforations in a predeterminedpattern if such be desired.

In the perforation of fabrics having coatings of enamel it is oftennecessary to make a slight rupture only in the surface of the enamel inorder to effect a very considerable reduction in dielectric strength.The reduction in dielectric strength is frequently out of all proportionto the depth of the incision.

By the term arc, as used herein, is meant an ionized path of a,relatively low electrical resistance extending continuously betweenelectrodes, in distinction to a brush or corona discharge or similarphenomenon. Such ionized path permits the passage of the relatively highcurrents which are necessary for producing perforations of the desireddiameter and performance.

It has been found possible, by using the method hereinabove described,to perforate material such as leatherette with a large number ofperforations per unit area and with the material moving through theapparatus at a fairly high rate of speed. By methods heretofore used itwas not possible, or at least practical, to electrically perforatematerials of this character. A few scattered perforations may be made,but it has not been found possible (except, perhaps, by a very slowoperation) to form enough perforations to provide, say, any effectiveVentilating function.

. With applicants present invention it is possible to make as many assixty-four or more perforations per square inch in material of thissort. If leatherette is formed with say from sixteen to sixty-fourperforations per square inch, the perforations Will have an appreciableVentilating function. The size of the perforations, on the average, mayvary from two to four thousandths of an inch in diameter. These guresare. of course, purely illustrative. The perforations may be minuteenough so that water will not pass therethrough except underconsiderable pressure. When desired, they may be so small as to bepractically invisible to casual observations at least. The perforationsmay be large enough or numerous enough to have an effective Ventilatingfunction without appreciably affecting the strength of the sheetmaterial.

We claim: g

1. Apparatus for electro-perforation of dielectric sheet material whichcomprises in combination: rotatable pin-pricking devices; a plurality ofpairs of electrodes providing spark gaps; means for moving said sheetmaterial in contact with said pin-pricking devices, to form pointincisions therein, and thereafter for moving the material through saidspark gaps; and means for causing high potential electric currents todischarge across said spark gaps and through said material at said pointincisions.

2. Apparatus for electro-perforation of dielectric sheet material whichcomprises in combination: rotatable pin-pricking devices; a plurality ofpairs of electrodes providing spark gaps substantially in line with saidpin-pricking devices, respectively; means for moving said sheet materialin contact with said pin-pricking devices, to form point incisionstherein, and thereafter for moving said material through said sparkgaps; and means for causing high potential electric currents todischarge across said spark gaps and through said material at saidpoints of incision.

3. Apparatus for electro-perforation of dielectric sheet material whichcomprises in combination: a plurality of series of sets circularlyarranged, rotating pin-pricking devices; a plurality of pairs ofelectrodes forming spark gaps which are in substantial alignment withsaid series of pin-pricking devices, respectively; means for moving saidsheet material in contact with said pin-pricking devices, to form pointincisions therein, and then through said spark gaps; and means forcausing electrical discharges to arc across said spark gaps and throughthe material at said points of incision.

4. Art of obtaining closely spaced perforations in sheet material havinghigh dielectric strength which comprises reducing the dielectricstrength of the material at a plurality of predetermined points bypenetrating the sheet from one surface a distance less than thethickness of the sheet whereby the dielectric strength of the sheet isweakened and the tendency, as the sheet is subsequently moved througharcs, for the discharges to pass through perforations already madeinstead of making new perforations, is prevented, and substantiallyimmediately thereafter causing electrical discharges to arc -through thematerial at such points to form electric arc made perforations.

5. Art of obtaining closely spaced perforations in sheet material havinghigh dielectric strength which comprises reducing the dielectricstrength of the material at a plurality of predetermined points bymaking point incisions in the sheet from one surface a distance lessthan the .thickness of the sheet whereby the dielectric strength of thesheet is weakened and the tendency, as the sheet is subsequently movedthrough arcs, for .the discharges to pass through perforations alreadymade instead of making new perforations, is prevented, and substantiallyimmediately thereafter causing electrical discharges to arc through thematerial at such points to form electric arc made perforations.

JOHN W. MEAKER. EDWARD H. YONKERS, JR.

